1. Field of the Invention
The invention relates to a process for evaporating constituents of a liquid by passing alternating current through the liquid.
2. Description of the Related Art
In the process technology sector, it has been known for some time that, for an efficient heat transfer via hot surfaces, the temperature difference between heating surface and liquid to be heated must be at a maximum. However, problems arise when pursuing this maximum in the case of heating of liquids comprising gaseous and/or low-boiling components. When the surface temperature exceeds a critical temperature difference from the boiling point of the low-boiling component, a vapor film thus forms at the heating surface, which thermally insulates the liquid from the heating surface and therefore worsens the heat flow. This phenomenon is known as the “Leidenfrost phenomenon”. The necessary introduction of heat into the liquid in such cases can therefore be achieved only by increasing the heat exchange area. On the other hand, however, such an increase in the heating surface area is impossible or very expensive owing to the nature of the apparatus and process prerequisites.
Alkylchlorosilanes are prepared by the route of the so-called direct synthesis from Si and MeCl. This affords a complex mixture of different alkylchlorosilanes with different boiling points. The target product is dichlorodimethylsilane with a boiling point of 71° C. (1013 mbar). In the distillative recovery of the pure alkylchlorosilanes from the product mixture obtainable by the direct synthesis, distillation residues with a boiling point of >71° C. are obtained. These are complex substance mixtures which contain compounds with SiSi, SiOSi, SiCSi, SiCCSi and SiCCCSi structures.
The composition of these so-called “high boilers” is described in detail, for example, in EP 0 635 510. As a result of the raw material or of the selective addition of catalytically active constituents, not only Si, but also further impurities of Cu, Zn, Sn, Al, Fe, Ca, Mn, Ti, Mg, Ni, Cr, B, P and C are found in the product stream of the direct synthesis. The impurities are present in suspended or dissolved form. The dissolved impurities are usually chlorides.
To heat the distillation residues, according to the prior art, for example, circulation evaporators, thin-film evaporators, short-path evaporators or heat exchangers are used. After removal of the lower-boiling components, in which the above problems exist in heat transfer, however, the thermal stability of the liquid constituents still present decreases under the influence of heat, and in the presence of the suspended or dissolved impurities. This results in oligomerization and polymerization reactions. The viscosities of the mixtures increase. This results in deposition of undesired deposits in pipelines, and in particular on the hot surfaces of the evaporators employed. Mass and energy transfer is increasingly hindered. Owing to heat transfer which has been reduced as a result, the surface temperature of the evaporator surfaces has to be increased further, which in turn leads to accelerated coverage thereof. As a result, the apparatus has to be cleaned often, the distillate yield falls, and the plant availability is unsatisfactory.
A disadvantage of the above-described prior art processes is the principle of heat introduction. Heat is transferred to the liquid silanes via hot surfaces, for example metals or graphite, which are in turn heated by heat carriers (steam, heat carrier oil) or electrical heating elements. In the case of this type of heat transfer, the surface temperature of the heat-transferring medium must be higher than the liquid to be heated. These higher surface temperatures are the cause of the problems described.